Periodically I am asked whether it’s “safe” to leave a computer on all the time. The answer is yes if it’s a desktop (meaning not a laptop).
Chances are if you’ve been using computers long enough you’ve encountered at least one instance where some type of hardware failure occurred right as you turned on the box – and I’m betting it had something to do with a part that continually rotates when active.
Continually rotating parts in a computer are fans and hard drives (on the inside), and the most “grunt” needed is when they spin up from an absolute stop. When spinning they don’t have to work as hard.
To note: A DVD drive does not continually rotate. This is because when not using it, it doesn’t spin at all whatsoever even when the box is on.
Concerning hard drives:
It’s a good bet if you’ve ever seen a hard drive failure, it probably happened from a “cold” start, then the infamous “disk not found” message appeared.
Fans accumulate dust whether you want them to or not. This adds weight to the fan blades and can also dry out the bearing(s). If the fans stay spinning they will continue to do so almost indefinitely. However if they’re old and clogged up with dirt (including the dirt you can’t see or clean out), one day they simply won’t spin up from a cold start at all.
Starting up a hard drive from an absolute stop requires the most effort from it just to get it spinning – and this follows suit with coolant fans.
I sincerely do subscribe to the theory that having the computer on all the time is safe and does make it last longer.
In the way I have my personal desktop box set up, I specifically set the hard drives to never “sleep” because it’s essentially the same as starting a drive cold.
I want to note this is my personal opinion on whether leaving a computer box on all the time is safe or not. Based on my personal experience, anything the moves in a computer is better off if you keep it moving and helps to prevent premature failure.